The goal of this proposal is to evaluate the feasibility of blocking interactions between the PD-1 immunoinhibitory receptor and its ligands as a strategy to enhance protective antibody responses to pneumococcal polysaccharides (PPSs) found in the capsule of Streptococcus pneumoniae. S. pneumoniae continues to pose a global threat and is one of the leading causes of morbidity and mortality among children. PPS vaccines provide significant protection against serotype-specific invasive pneumococcal disease in adults, but are less efficacious against non-invasive disease and poorly stimulate protective antibody responses in very young, elderly, and immunocompromised populations. Understanding how protective antibody responses to PPS are regulated is key to devising more efficacious vaccines. PPSs typically elicit T cell independent type 2 (TI-2) antibody responses, which are regulated distinctly from antibody responses to protein antigens. We previously identified that a unique subset of B cells, termed B-1b cells, is critical for producing antibodies against type 3 PPS (PPS-3) and other TI-2 antigens in mice. Our preliminary data shows that antigen-specific B-1b cells upregulate the immunoinhibitory receptor PD-1 following TI-2 antigen immunization, and that blocking PD-1 from interacting with its ligands significantly increases IgG responses against TI-2 Ags, including PPS-3. Despite the intense interest in developing PD-1-related biologics and small molecule inhibitors for treatment of chronic viral infections and malignancies, very little is known about how these therapies may influence B cell function or host defense against bacteria. Studies in this proposal are designed to explore: 1) how inhibiting PD-1 function influences the quantity, persistence, and protective capacity of antibody responses to PPS-3, 2) the extent to which PD-1 regulates antibody responses to distinct types of PPS found in the current pneumococcal vaccine, and 3) the role B-cell-specific PD-1 expression plays in the regulation of PPS antibody responses. Finally, since PD-1 expression is only significantly induced on antigen-specific B-1b cells following TI-2 antigen immunization in mice, we will explore the functional contribution B-1-like cells make to TI-2 antibody responses in non-human primates (African Green monkeys; AGM) and the potential for PD-1 to similarly regulate these cells. The results of these exploratory studies will demonstrate the significance the PD-1 regulatory pathway has on the generation of protective humoral immunity to S. pneumoniae and will reveal the importance B cell-specific PD-1 expression plays in this process in mice. Finally, this project will further develop the AGM as an alternative non-human primate model in which to assess B cell subsets and factors contributing to the regulation of antibody responses against PPS and other clinically relevant TI-2 antigens. Ultimately, these studies may lead to new applications for PD-1 therapeutics in enhancing the efficacy of PPS-based vaccines against S. pneumoniae. PUBLIC HEALTH RELEVANCE: The goal of the research proposed in the current application is to understand how the immune system is regulated to respond to vaccines against Streptococcus pneumoniae. The knowledge gained from these studies may lead to improved vaccines that provide enhanced protection against pneumococcal infections currently threatening global public health.